Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Note Receivable [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2018	$ 14,738	$ 51	$ 389,398	$ (16)	$ (198)	$ (374,497)
Beginning Balance, shares at Dec. 31, 2018		50,652,238
Net loss	(39,961)	$ 0	0	0	0	(39,961)
Unrealized (loss) gain on investments	607	0	0	0	607	0
Issuance of common stock under Equity Distribution Agreement, net of issuance costs	73,276	$ 6	73,270	0	0	0
Issuance of common stock under Equity Distribution Agreement, net of issuance costs, shares		6,491,196
Issuance of common stock under equity incentive and employee stock purchase plans	5,837	$ 1	5,836	0	0	0
Issuance of common stock under equity incentive and employee stock purchase plans, shares		1,217,325
Repurchased shares upon vesting of restricted stock units for tax withholdings	(1,174)	$ 0	(1,174)	0	0	0
Repurchased shares upon vesting of restricted stock units for tax withholdings, shares		(107,619)
Employee stock-based compensation	8,359	$ 0	8,359	0	0	0
Compensation expense related to options granted to consultants	180	0	180	0	0	0
Ending Balance at Sep. 30, 2019	61,862	$ 58	475,869	(16)	409	(414,458)
Ending Balance, shares at Sep. 30, 2019		58,253,140
Beginning Balance at Jun. 30, 2019	71,460	$ 58	472,661	(16)	353	(401,596)
Beginning Balance, shares at Jun. 30, 2019		58,209,745
Net loss	(12,862)	$ 0	0	0	0	(12,862)
Unrealized (loss) gain on investments	56	0	0	0	56	0
Issuance of common stock under equity incentive and employee stock purchase plans	269	$ 0	269	0	0	0
Issuance of common stock under equity incentive and employee stock purchase plans, shares		43,395
Employee stock-based compensation	2,856	$ 0	2,856	0	0	0
Compensation expense related to options granted to consultants	83	0	83	0	0	0
Ending Balance at Sep. 30, 2019	61,862	$ 58	475,869	(16)	409	(414,458)
Ending Balance, shares at Sep. 30, 2019		58,253,140
Beginning Balance at Dec. 31, 2019	$ 66,000	$ 60	495,624	(16)	318	(429,986)
Beginning Balance, shares at Dec. 31, 2019	60,234,784	60,234,784
Net loss	$ (25,480)	$ 0	0	0	0	(25,480)
Unrealized (loss) gain on investments	(46)	0	0	0	(46)	0
Issuance of common stock upon follow-on offering, net of issuance costs	325,654	$ 6	325,648	0	0	0
Issuance of common stock upon follow-on offering, net of issuance costs, shares		5,980,000
Issuance of common stock under equity incentive and employee stock purchase plans	26,498	$ 3	26,495	0	0	0
Issuance of common stock under equity incentive and employee stock purchase plans, shares		2,986,248
Repurchased shares upon vesting of restricted stock units for tax withholdings	(3,709)	$ 0	(3,709)	0	0	0
Repurchased shares upon vesting of restricted stock units for tax withholdings, shares		(92,459)
Employee stock-based compensation	15,171	$ 0	15,171	0	0	0
Compensation expense related to options granted to consultants	731	0	731	0	0	0
Ending Balance at Sep. 30, 2020	$ 404,819	$ 69	859,960	(16)	272	(455,466)
Ending Balance, shares at Sep. 30, 2020	69,108,573	69,108,573
Beginning Balance at Jun. 30, 2020	$ 421,141	$ 69	852,006	(16)	488	(431,406)
Beginning Balance, shares at Jun. 30, 2020		68,803,400
Net loss	(24,060)	$ 0	0	0	0	(24,060)
Unrealized (loss) gain on investments	(216)	0	0	0	(216)	0
Issuance of common stock under equity incentive and employee stock purchase plans	2,005	$ 0	2,005	0	0	0
Issuance of common stock under equity incentive and employee stock purchase plans, shares		305,173
Employee stock-based compensation	5,721	$ 0	5,721	0	0	0
Compensation expense related to options granted to consultants	228	0	228	0	0	0
Ending Balance at Sep. 30, 2020	$ 404,819	$ 69	$ 859,960	$ (16)	$ 272	$ (455,466)
Ending Balance, shares at Sep. 30, 2020	69,108,573	69,108,573